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20: USER-ADDED (.TDF) DATASETS      

   As delivered,  Guide displays an extremely wide variety of datasets
covering many kinds of celestial objects.  However, some people will
have the need or desire to add objects from completely separate
datasets previously unknown to Guide.  The user-added dataset
capability lets you do this.  It also allows you to display and get
information from a long list of datasets already created by others,
such as quasars,  pulsars,  double stars,  and more. In fact,  people
usually use the datasets already "added" by others, only occasionally
getting around to adding their own.  We'll describe first how to use
these datasets,  before explaining how to add new ones.

   The "Toggle User Datasets" option in the Extras menu controls the
display of these pre-defined datasets. When you first use this, only
the pre-defined datasets will be shown in the list box, and you can
turn them on or off,  select colors,  and tell Guide at what fields
of view and to what magnitude they should be shown.  Once displayed,
you can right-click on the objects and get "more info" data,  just as
with any other dataset.  (As with many other types of objects shown
on Guide's charts,  you can also right-click on them and then on
"display",  then turn them on or off,  etc.)

   You can also find objects in these datasets using the "Go to .TDF
Object" option described on page 8.

   And now for a discussion of how to add your own datasets.  The basic
idea is a pretty simple one.  Most databases are in plain ASCII text or
FITS files,  with data arranged in columns. Guide will absolutely need
to know certain basic things about the database,  such as which columns
contain the RA hours data, which the declination minutes, the file name
of the database,  the epoch of the coordinates,  and so on.

   All of this information is stored in a Text Definition File (.TDF).
Four examples are provided in your Guide directory:  CD_DATA.TDF,
CD_DATA2.TDF,  CD_DATA3.TDF,  and RADIO.TDF.  Each contains definition
data for several datasets on the Guide CD.  More are posted from time
to time on the Project Pluto Web site.

   Unfortunately,  the .TDF format is quite Guide-specific, because there
are no real "standards" in this area.  FITS files were a possible
standard,  but are (despite the name) far from standardized,  are hard to
edit using standard text editing tools, and lack many useful features.

   One can freely add new .TDF files;  Guide will automatically detect
them and display the data defined by them.  You can move datasets freely
from one .TDF file to another, or combine them all;  the default four
.TDFs just made dataset management a little simpler.

   If you edit either .TDF file with a text editor,  the format used
will probably be quite clear.  There are a lot of fine points to be

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